Hydraulic systems for controlling the acceleration, velocity and deceleration of heavy loads and in particular swing drives, have typically made use of some form of pressure control.
An excellent example is the closed loop swing drive pump control described in Vickers U.S. Pat. No. 3,696,836 which provides true pressure control in both driving and braking mode. In neutral, this control provides for free coast, a characteristic very desirable for construction cranes.
In other applications, like excavators, backhoe-loaders and forestry equipment, like log loaders and feller-bunchers, the free coast is not acceptable--neutral lever position must give blocked port conditions.
In these applications, the most common method of deceleration or braking is to center the valve and utilize either port relief valves or cross port relief valves. Improvements to the above have been made by introducing two relief valve pressure levels (one for acceleration, one for deceleration), or pressure rate sensing relief valves to smooth the action. Nevertheless, the one large performance disadvantage with the above systems is when once the valve is centered, there is no control of where the swing will stop. The stop position depends on load inertia. Another disadvantage is that there is no way to stop the swing motion earlier, if desired.
Among the objectives of the present invention are to provide a hydraulic system wherein the swing motion of the device being controlled is controlled throughout the desired movement; wherein both acceleration and deceleration may be controlled by one input signal from the same controller; wherein both acceleration and deceleration of large inertia loads, for example swing drives, can be achieved; wherein control of acceleration and deceleration is achieved by controlling both accelerating pressure and decelerating pressure simultaneously; wherein the system functions on the basis of a difference between acceleration or driving pressure and deceleration or braking pressure; and wherein the pressure controlling the swing motion comprises the only actuator connection to tank.
In accordance with the invention, the hydraulic control system comprises a hydraulic actuator having opposed openings adapted to alternately function as inlets and outlets for moving an element of the actuator in opposite directions, a pump for supplying fluid to the actuator. A meter-in valve individual to each opening is provided to which fluid from the pump is supplied and a pair of lines extends from the respective meter-in valves to the respective openings of the actuator. A controller alternately supplies pressure to the respective meter-in valves for controlling the direction of movement of the load. A second pair of lines extends from the first pair of lines to tank and a variable relief valve is positioned in each of the second pair of lines. When the controller is moved to actuate one of the meter-in valves for supplying fluid to one of the openings of the actuator, the controller also actuates the variable operated relief valve associated with the line of the first pair of lines extending from the other of the openings of actuator to control the flow out of the other opening. The controller thus simultaneously controls the fluid flow and pressure to the actuator and controls the fluid pressure from the actuator thereby simultaneously controlling the driving and braking functions of the system.